This invention relates to a position control apparatus for stopping or positioning the spindle of a machine tool or the like at a predetermined rotational angle. More particularly, the invention relates to a position control apparatus capable of enhancing servo rigidity when a spindle is stopped.
To stop a spindle at a predetermined rotational angle in an NC machine tool, by way of example, an error quantity, which is the difference between a commanded position and the present position of the spindle, is subjected to a digital-to-analog conversion by a DA converter to obtain a velocity control command signal, and the velocity of a motor for rotating the spindle is controlled by the control command signal.
In performing such position control, the gain of a velocity loop is made higher at the time of orientation (when the spindle is brought to a stop at a reference position) than at the time of ordinary rotation, thereby enhancing servo stability at the stopping time to increase the holding force.
In the prior art, gain changeover timing is taken at the instant at which a certain angular range is reached, such as a half revolution short of a target stopping position for orientation, and control is exercised in such a manner that the gain of a velocity amplifier is raised when the point one-half revolution short of the target stopping position is reached. In this way about twice the servo rigidity can be obtained in comparison with a case where orientation is performed at the gain which prevails during ordinary rotation.
However, the gain of the velocity amplifier is affected by the size of the applied velocity command signal. When the gain is raised while the velocity is high, as at the point one-half revolution short, the operating characteristic of the velocity amplifier becomes unstable. Consequently, the gain cannot be raised very much. In other words, a required, satisfactory servo rigidity cannot be obtained.